A conventional RFID system is illustrated in FIG. 1. The RFID system 100 comprises tags 110, a reader 120, radio frequency (RF) antennas 130, and a host 140 for processing identification signals. Non-contact data transmission between the tag 110 and the reader 120 is achieved by using RF signals. A communication protocol, for an example, the electronic product code (EPC) is a common transmission specification of such non-contact data transmission. In the communication protocol of the EPC, the reader 120 finds every tag 110 within a certain range and accesses content of the tags 110 according to a Q-algorithm or an anti-collision searching method. The reader 120 generates a Q value, and sends a query command having the Q value to every tag 110 within the range. The tag 110 randomly generates a count value according to the Q value when receiving the query command. Provided that when the generated count value is equal to 0, the tags 110 will then respond to the query command of the reader 120; and when only one of the tags 110 is responsive, the reader 120 transmits data to such tag 110. However, when more than one of the tags 110 simultaneously respond to the query command of the reader 120, the reader 120 again sends a command for resetting the Q value until that a count value of only one of the tags 110 is equal to 0.
In the foregoing Q-algorithm, as a result of all of the tags 110 having a same Q value and a same random procedure, the probability of randomly generating the count value of zero is the same. In other words, the probability of every tag 110 being accessed is the same. However, when there is one particular tag 110 carrying urgent information which needs to be accessed with a higher priority, the conventional Q-algorithm can not ensure that this tag 110 is accessed as quickly as possible.